A man has died after a fire ripped through a suburban unit south of Sydney.

The fire was reported in the unit in Kelly St, Berkeley, in the southern suburbs of Wollongong, NSW, about 9.25am on Monday.

An off-duty firefighter, who was walking with his wife, noticed the flames and kicked in the door, ABC News Illawarra reported.

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However, the fire was too fierce for him to go inside and he alerted fire crews, ABC said.

Fire and Rescue NSW attended and extinguished the fire, which caused only minor damage to the unit.

NSW Ambulance paramedics then found the body of a man, believed to be aged in his 40s, inside the single-level unit.

He has yet to be formally identified, NSW Police said.

No one else was injured, and adjoining units were not affected.

However, it is not yet known how the fire started and it is currently being treated as suspicious.

“Investigations have commenced into the circumstances of the fire,” police said.

Authorities are urging anyone with CCTV footage or information to contact police or Crime Stoppers.